Sand Filter Treatment - Requirements For Wastewater
The following are the slow sand filter design requirements including the recommended standards of performance:
a) The filtrate used in sand filter treatment should be clear with turbidity in the range 1 ppm (1 JTU - Jackson Turbidity Unit) to 3 ppm.
b) The filtrate used in sand filter treatment should be color-free and read below 3 in the cobalt scale.
c) With the raw water turbidity inside 30 ppm (30 JTU), the filter run should exceed 6 to 8 weeks with the filter head loss below 0.6 m.
d) If the initial loss of head exceeds.02 to.05 cm, the entire sand bed needs overhauling.
The relative advantages and disadvantages of slow wastewater sand filters over rapid sand filters are:
* Can be without coagulation.
* Simple design for the equipment.
* Suitable sand will usually be readily available.
* Simple supervision is possible.
* Less corrosive and more uniform effluent.
* Effective removal of bacteria.
* Size is bigger, so cost is more.
* Operational flexibility is less.
* If not accompanied by preliminary plain sedimentation, not economical for raw waters with turbidity above 30.
* Color removal is less effective during treatment.
* If thre is no pretreatment, then there are poor results for water with high algae content.
* Complicates effective feeding, mixing and flocculation of water to be filtered.
* It may not be possible to provide adequate chlorine contact time, if supply is direct from pressure filter.
* It is not possible to observe the effectiveness of the backwash, since the water, sand bed etc., are not in sight.
* Effective design of wastewater sand filter gutter is impossible due to the inherent shape of the pressure filter.
* To inspect, clean and replace the sand, gravel and under drains of pressure filters is difficult, since access is not easy.
* Sometimes, results can be disastrous due to sudden pressure differentials of the two sides of the filters.
* Condition of the sand bed may be difficult in ascertaining.
* Pressure filters are used only for small industrial needs and swimming. This is because, the disadvantages outweigh the advantages.
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